The symptoms of political amnesia, an ailment that can detract from a sufferer's electoral appeal, were evident in ACT leader John Banks' struggles to recall his dealings with donors to his 2010 Auckland mayoral campaign. This week they became evident, too, in National leader John Key's forgetting aspects of a briefing by the spy chiefs who report to him.

It is too soon to say if an epidemic has broken out within the Key Administration or who might be susceptible. This might be an ailment of concern only to politicians named John, or some party leaders, or some with a ministerial warrant.

But it is apt to weaken the credibility of those who have been infected and this could erode support for their parties at the next election.

Mr Banks' memory frailties have been exposed on more than occasion. In April, for example, the Campbell Live programme broke the story of Kim Dotcom and his controversial donations. It also provided evidence of Mr Banks' helicopter ride to lunch with Mr Dotcom at his Coatsville mansion, a happening the minister could not recall.

Further evidence of Mr Banks' relationship with Mr Dotcom has emerged since, prompting calls for his resignation from Parliament. He has stayed on and the prime minister has not sacked him, perhaps because he appreciates how easy it is to forget things.

Mr Key has admitted not being able to remember how he felt about the 1981 Springbok tour or about the country being declared nuclear-free, but those events happened long before he entered politics.

His recent lapse is less understandable. Last week he said he had been informed about the Government Communications Security Bureau's illegal spying on Mr Dotcom only in recent weeks. This week he acknowledged he had sat through a briefing on the Dotcom case in February, not long after the high-profile raid, but somehow he had forgotten about it.

American writer Elbert Hubbart said "the ability to forget is the true token of greatness". By that measure we are blessed with great leaders. If we subscribe to a different view, however, we should not forget these memory failures when we cast our votes at the next election.